[llvm] [AMDGPU] Document GFX12 Memory Model (PR #98599)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

note: I plan to include the code changes in this diff as well and just make it a "finalize GFX12 memory model" patch. It makes more sense and will be easier to review like that IMO.

I need a bit of time to finalize the code changes, they'll come later today or tomorrow.

https://github.com/llvm/llvm-project/pull/98599